Decaf is short for decaffeinated coffee. Ever had coffee and lost your sleep well that's because of caffeine coffee by weight has a small amount of caffeine but that small amount of caffeine by weight is enough to keep us up all night if we have a cup at night.  

Now decaffeination is a process where this caffeine is actually removed from the green coffee bean before it's been roasted in fact caffeine reduction is all the way up to 97 to 98 percent in the green beans. So, you just have one to two percent of caffeine left in the green bean speaking of green coffee arabica has half the amount of caffeine as compared to Robusta so decaffeination is usually done with arabica because it even has lower amounts of caffeine than Robusta.  

If you're sensitive to caffeine choose arabica over Robusta and if you're even more sensitive to caffeine, choose decaf or arabica.